How deep sea aircon could cut the heat of climate change

The deep ocean is cold; our cities are growing increasingly warm. What if we could tap those frigid depths to cool down energy-hungry metropolises?

Until the 1960s, when jet travel brought mass tourism to Hawaii and skyscrapers began sprouting from Waikiki to Pearl Harbor, Honolulu was a city of widely spread-out low buildings shaded by giant monkeypod trees naturally cooled by the strong trade winds that prevail 2000km north of the equator.

Today, however, a forest of concrete structures that not only trap heat but emit it through air-conditioning have turned the city center into a heat island and have become a drain on the state’s over-extended power-generation system.

In the next few months, work will begin on a project to make Honolulu the first city in the world to have its heart cooled by seawater pumped from the deep. This will save building owners money in several different ways and at the same time cut greenhouse emissions, according to an environmental impact statement published last fall.

The technology is already used in parts of Toronto and Stockholm to balance temperatures inside large buildings, notably to cool areas with computer servers and telephone exchanges during the summer months. But, says William M. Mahlum, president of Honolulu Seawater Air Conditioning LLC, which is undertaking the $240-million project, 'this is the first time it will be used to cool a warm-weather city center.'

A sea of history

It’s not surprising this first came about in Hawaii. It was on the Big Island in 1974 that the Natural Energy Laboratory of Hawaii Authority (NELHA) began the first US tests of Ocean Thermal Energy Conversion (OTEC) technology to see if temperature differences between the deep and the surface could be turned into electricity economically.

In 1979, a barge dubbed 'Mini-OTEC', anchored off Keahole Point, demonstrated that OTEC was indeed a viable energy source, producing a net 15 kW of eletricity by exploiting the temperature differential between water from 670 metres at 5.6°C, and surface water at 26°C.

The downtown area to be connected to the seawater air conditioning system, courtesyof Honolulu Seawater Air Conditioning LLC

The technology was simple and robust (it is still working today), but at the time did not produce electricity at an economically viable price. A later project set about pumping the cool water through heat exchangers, and the notion of ocean-fed air conditioning was born.Saving energy; reducing pollution

Ground is expected to be broken this summer and the first 40 buildings are expected to come online in late 2012. Another five will be added the following year.

The system will save its clients about 20 per cent in cooling costs (based on oil prices at $100 a barrel), the company says, and by reducing power use by 77 million kWh/year, or 75 per cent of the present consumption, it will cut carbon dioxide emissions by 84,000 tons a year, nitrogen oxides by 169 tons and sulphur oxides by 165 tons. It will also reduce the use of refrigerants like HCFC-22, HCFC-123 and CFC-11/500 and save 260 million gallons per year of drinking water.

In addition, it will end the dumping into Honolulu’s sewers of 84 million gallons per year of used cooling tower water containing chemicals such as phosphoric and sulfuric acids, aromatic amines (for corrosion, scaling and fouling control), difluoro-ethane (microbiological control and insecticide), as well as chlorine and isothiazolin (microbiological control), according to Ingvar Larsson, Honolulu Seawater’s Vice President of Engineering.

'Most green energy projects focus on generating clean electrons,' says Jeff Mikulina, head of the Blue Planet Foundation, whose goal is to promote clean energy in Hawaii, which gets more of its power from burning oil than any other state. 'The beauty of this one is that it avoid electrons altogether by tapping into a vast local resource.'

A diagram of the proposed Honolulu system, courtesy of Honolulu Seawater AirConditioning LLC

The process

Here’s how it works: a five-foot-wide pipe extends four miles out to sea to a depth of 1,700 feet, bringing in 44,000 gallons of water every minute at 45°F (7.2°C). Once ashore, the water is fed into a conventionally powered chiller that brings it down to exactly 44°F (6.7°C), then flows through a heat exchanger with a closed-circuit freshwater system and is released back into the sea at a depth of 200 feet at 56°F (13.3°C) – the natural temperature at that depth.

Meanwhile, the cooled freshwater makes its way to the buildings’ air conditioning units and cools the air flowing over the chilled coils. This allows the building to turn off the energy-hungry compressors that previously did the same job.

Hawaii, like many warm climate locations, sees a surge of power demand in the middle of the day driven by air conditioning, so the ocean-fed system would not only save power, but premium power, which is often provided by the dirtiest power stations. 'This project will help relieve some strain on the power grid,' notes Mikulina.

Heat pollution?

One concern might be that the system would locally raise ocean temperatures - a problem that has been noted around the outputs from water-cooled power stations. Not in this case, says Ingvar Larsson, Honolulu Seawater’s vice president of engineering. 'If you look at the heat we emit in both the ocean and the atmosphere, it’s 40 percent less than a conventional air conditioning system, and of course by cutting greenhouse gases, we slow global warming,' he says.

An illustration of how the intake pipeslopes down the steep coastal shelf, courtesyof Honolulu Seawater Air Conditioning LLC

Even so, marine microbiologist David M. Karl warns that the water at depth contains more nutrients than the water at 200 feet, so releasing warm, nutrient-rich water at shallower depths could led to a rapid increase in biological activity.

'We’ll be monitoring to see if it creates an algal bloom,' adds Karl, who heads the Laboratory for Microbial Oceanography of School of Ocean and Earth Science and Technology at the University of Hawaii at Manoa.A nod from the grid

The project even has fans amongst conventional utilities, who are ultimately losing business as a result of its installation. Peter Rosegg, a Hawaii Electric Company spokesman, has nothing but praise for the project. 'By reducing our load, it allows to increase our reliability to other customers,' he says.

The system will reduce electricity generation in Hawaii by only one per cent – one year’s worth of growth in demand. But Rosegg says, 'one per cent may not seem like much, but this is an important one per cent because the downtown area has banks and medical centers that require very reliable power. They can’t afford dips and curls, and seawater cooling is much more even than, say, wind and solar power, which go up and down a lot.'

Future steps

The next logical place for an ocean-fed air conditioning system would be Waikiki, the Honolulu neighborhood just east of downtown, which has also become a Manhattan-like forest of skyscraper condominiums and hotels. Over one third of power use there is for air conditioning. Honolulu Seawater Air Conditioning officials confirm that once the current project is finished they expect to create another unit in Waikiki, with its own pipe into the ocean. This would take another five years.

Once the first system is up and running, however, it should inspire tropical coastal cities around the world to harness the technology, predicts Mahlum. 'All you need is a steep enough coastal gradient and concentrated demand,' he says. 'We've commissioned a study of U.S. cities and found that it would work in more than 30 of them.'

Just where those 30 cities are, however, is still 'commercially sensitive information'.

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